For present purpose “patient circuit” means an open or re-circulating conduit for transporting respiratory gas between a ventilator and a human or animal patient. “Tidal volume” is the volume of air inhaled and exhaled at each breath. “Deadspace” is a volumetric space in the flow sensor and associated tube connectors from which gas does not reach the patient's alveoli during inhalation and/or which buffers carbon dioxide rich gas from reaching the exhaust channel during exhalation.
Systems for measuring the gas volume flow and gas composition in respiratory therapy are well known. An example of a constant temperature anemometry flow sensor is described in U.S. Pat. No. 4,363,236, and an example of a gas sampling and analysis system is described in U.S. Pat. No. 7,341,563. Due to the fact that partial pressure of carbon dioxide in the fresh ventilation gas is practically zero, the partial pressure of carbon dioxide in the patient alveoli closely represents that in the patient blood system. Units of exhaled gas originating from the alveoli are generally the last to be washed-out, in the end-tidal volume. Other tidal gas inhaled into and out from the upper airway and lung branches does not reach the alveoli and does therefore not contain carbon dioxide that is exchanged in the current breath. Gas flow turbulence prior to reaching the side-stream sampling port causes the end-tidal gas volume to mix with other tidal gas. The effect is an under reading of the actual carbon dioxide value from the alveoli. This under reading effect can be considerable for premature and infant patients, where the end-tidal gas volume is very small. In addition to causing under-measurement of the indicator for blood carbon dioxide, the deadspace may also result in an actual increase in blood carbon dioxide, by detracting from the amount of fresh gas exchange and causing rebreathing of previously exhaled gas. One way to minimize deadspace is to ‘throttle down’ or reduce the cross-sectional area of the tubular bore through the flow sensor. However, this has effect of simultaneously increasing the gas flow resistance and inhibiting the effective emptying of the lung. The tube bore cross-section therefore has an optimum dimension which relates to the ventilation flow rate and volume, which in turn relates to the size of patient. Taking the smallest patients into account, the deadspace should ideally be less than 1 ml. An example of a gas sampling adapter arrangement for reducing deadspace and adverse gas mixing is described in U.S. Pat. No. 6,926,005 and in U.S. Pat. No. 7,059,322.
A further problem can exist caused by the diversion of gas flow for sampling as this can reduce accuracy in the volume flow measurement. The tidal volume in infants is 4-8 ml/kg weight. The smallest viable patients weigh down to about 300 g and may tolerate as little as 2 ml tidal volume, which at 50 breathes-per-minute equates to a volume of approximately 100 ml/min. In a severe case as described below in respect of the prior art shown in FIGS. 1 and 3 the diversion of sampling gas, on the patient side of the flow measurement site, can result in a 50% under-measurement of the actual tidal volume. Modern ventilators would register such loss as a leak at the patient interface. Ventilation is an output driven therapy, where the target tidal volume is, typically, adjusted according to patient response. On-going ventilator adjustments generally take into account and compensate for a certain amount of leak. However, the leak effect is a further level of complexity for the clinician to consider, and thereby introduces a risk of sub-optimal ventilator settings. Accidental over-distension (by too large a volume) may cause irreparable lung tissue damage. Often the patient's metabolic rate is calculated from gas volume and carbon dioxide and oxygen concentration variables. Such calculations are potentially unreliable when using leak compensated volume estimation. It is therefore desirable to eliminate tidal volume measurement errors and rogue leak measurements.